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DSE00 Mock 2017



Paper 1A        version 1.8             (31/3/2017)
Paper 1B        version 1.4             (31/3/2017)
Marking         version 2.1             (31/3/2017)
                       


1A 發問區 (下面)
1B 發問區 

Quick search:   Ctrl+G 搜尋題號 (e.g: Q1)

110 則留言:

  1. for question 2== why electrolysis of molten al2o3 produce oxygen?but not dilute hcl?

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    1. MCQ2)
      Dil HCl, Cl- lose electron instead of OH-. So Cl2 is given out.

      In molten Al2O3, O2- ion is mobile and lose electron and become O2

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    2. but in dse, only concentrated hcl will give of cl2 gas ??....

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    3. in DSE, only very dil HCl will give O2.

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    4. Molten Al2O3 only contain Al3+ ions and O2- ions, nothing else.

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  2. q9 fe3+ + 3oh- to feoh3 isnt the ionic equation correct? so why the ans is b?

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    1. MCQ9)
      you should mind that [Fe3+] in 2 M Fe2(SO4)3 is 4 M.

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    2. so why the ans isnt 6.408g?

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  3. q34 y the polymer has giant cobvalent structure?

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    1. MCQ34)
      The question asked is "incorrect"

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    2. why is it not chemically inert?

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    3. nvm I get la, thx

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  4. for q19== u said which of the below can be identify by br2only== isnt the hexene can turn acidified kmno4 from pink to colorless and thus it can be identify by kmno4 also? and why option 3 is correct????

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    1. MCQ19)
      hexene can turn KMnO4/H+ colourless immediately while hexane cannot.

      Br(aq) = Br2 + H2O. Ca react with water while Mg doesnt.

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    2. as u said , hexene can turn KMnO4/H+ colourless immediately while hexane cannot. it isnt can be distinguished by using Br2(aq) ONLY , SO shouldnt option 1 is wrong?

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    3. Oh sorry, what I mean is hexene can turn Br2 colourless immediately (addition) while hexane cannot.

      PS: hexane also can undergo substitution with Br2 under UV light but the process is much slower then addition

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    4. so we still can distingilsh it by Br2 to see it is hexane or hexene in dark , right?? thankyou

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    5. Yes, hexene (all C=C ) react with Br2 very quict even though in dark.
      But C-C (alkane) can't

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    6. Hi,
      I would like to ask Q19. The question is "which following pairs of substance can be distinguished by using Br2 only"

      However, you do not give any conditions. Therefore, can I assume that the reaction undergoes at the presence of light?

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    7. MC 19
      The word "only" is used in the question.
      That means the pairs of substances can use Br2 to distinguish only without other methods.
      Since hexane and hexene also can be distinguished by KMnO4/H+. Why option 1 is correct?
      Thanks

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    8. Thanks for your suggestion. Removing "only" is indeed more appropriate.

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    9. For option 3, I know that Mg will not react with cold water. But however, with reference to the Electrochemical Series, both Ca and Mg is a strong reducing agent, therefore both of them can react with Bromine water, which is a strong oxidizing agent?Therefore option 3 should be wrong?

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    10. yes, both Mg and Ca can turn Br2 to Br- which is colourless. But Ca react with cold water and Br2 in the same time. So

      Ca: colourless bubble + solution become colourless.
      Mg: solution become colourless.

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    11. why Ca can react with cold water but Mg can't?

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  5. For question 2, decomposition of nitric acid would give oxygen ( 4 HNO3 → 2 H2O + 4 NO2 + O2 ), right?

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  6. Why CH2=CHCH2CH2CH3 has 5 structural isomers ?

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    1. MCQ26)
      Before you draw, you should mind that geometrical isomers are not included.

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    2. What are the 5 isomers? I cannot draw them, thanks.

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    3. 2-methylbut-1-ene
      3-methylbut-1-ene
      2-methylbut-2-ene
      pentene
      pent-2-ene

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  7. Q6
    the change in oxidation number should be -2 → +4 → +4 → +6 → 0
    shouldn't the answer be B instead of C?

    Q20
    Decomposition of nitric acid 4 HNO3 → 2 H2O + 4 NO2 + O2
    shouldn't volume decrease over time?

    Q23
    tasting the sample may not be a very wise choice
    but I certainly can distinguish between table salt and sugar by tasting
    is there any reason behind?

    thx for sharing and answering =]

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    1. MCQ6)
      Oh, this is my mistyping.

      MCQ20)
      HNO3 is a solute dissolved in water. Decrease of solute will not affect the volume of solvent. On the other hands, water is produced. The volume would likely increase instead.

      MCQ23)
      Even though we know they are different taste, we never taste any chemical in any experiment because of safety reason.

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  8. q5 can you explain why answer is b?

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    1. MCQ5)
      Phenolphthalen is pink if OH- here
      K3Fe(CN)6(aq) is blue if Fe2+ here.
      But Zn + Fe2+ --> Zn2+ + Fe
      so only OH- and Zn2+ are here and the color is pink

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    2. so the present of oh- and zn- is because the zn react with h2o?

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    3. Not Zn react with H2O. Only K, Na, Ca can react with water.
      Fe react with H2O and O2 (corrosion) and form Fe2+ + OH-.
      Zn donate its electron to Fe2+ (sacrificial protection) and
      become Zn2+ + Fe.

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    4. Mc5
      As you say, K3Fe(CN)6(aq) is blue if Fe2+ here. However, Since there is no presence of Fe2+ ions and K3Fe(CN)6(aq) is yellow in colour. Why D is incorrect?
      Thanks

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    5. Thanks for your suggestion. let me amend the question. =]

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  9. can you explain q29?

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    1. MCQ29)
      it is an esterification.
      the single bond O part come from alcohol.
      after break the O-O bond and add "H atom" to single bond O,
      the structure of alcohol can be found.
      H2SO4 is used as catalyst

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    2. should the H2SO4 be conc. instead of dilu. ?

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    3. theoretically, H+ can catalysis the reaction whatever it is conc. or dil.
      But conc. H2SO4 can shift the eqm position to product (ester) site.

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  10. shouldnt tras have a hight melting pt than cis?
    thx

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    1. yes, trans has higher melting point but lower boiling point than cis

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  11. q18 isnt (3) is wrong? from gas to liquid should release heat thus the Enthalpy change should be negative?

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    1. MCQ18)
      yes, (g) to (l) is an exothermic reaction and then a negative value. So (3) is wrong and the answer is B-- (1) and (2) are correct

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  12. q15 (a) can you name some exception?

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    1. MCQ15)
      NH4+ + Cl- --> NH4Cl
      NH4+ + OH- --> NH4OH
      none of N, H Cl O is metal element but they can form ionic compound

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  13. q12 u said enthalpy change but not standerd enthalpy change , so not a must to produce only 1 mol h2o ,thus (III) should be right?

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    1. MCQ12)
      yes you are right. I will add "standard" then =]

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  14. can you explain q10?

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    1. MCQ10)
      cut the chain every 2 carbon. You can see the monomer is methylpropene.
      "2-methylpropene" is wrong for naming. no "2" is need for methylpropene.

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  15. q16 why a is wrong?? thankyou =)

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    1. MCQ16)
      beware that 0.25 M of Fe2(SO4)3(aq) = 0.5 M of Fe3+

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    2. I don't understand. Isn't that the equation is 2Fe3+ + 2I- -> 2Fe2+ + I2?So why not 1:1?

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    3. the number of mole of Fe3+ = I-
      then [Fe3+] decrease is the same
      but [Fe2(SO4)3] decrease is the half
      because 1 Fe2(SO4)3 = 2 Fe3+

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  16. For Question 15, strong ionic bonds should exists between ions? So option D is wrong also?

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    1. MCQ15)
      because the statement is "between atom"

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    2. There are atoms between ionic compound?

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    3. no atom. because all atoms are changed to ion.
      (e.g Na+, Cl-) they are sodium ion and chloride but not
      sodium atom and chlorine atom anymore

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  17. For Question 34, why (1)(3) are incorrect?
    For Question 36, why statement 1 is wrong?
    Thank you:)

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    1. Q34)
      (1) the ester bond will react with acid
      (2) acid can break the ester bond but not the carbon chain

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    2. MCQ36
      10g O2 and 10g N2 has same density but different pressure.
      density = mass/volume
      they have same volume and same mass so they have same density

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    3. A little bit tricky for Q36 XD Then the statement is correct if 'in the same fixed volume' is deleted?

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    4. yes, maybe I should correct it to "two separated closed system with same volume"

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    5. So the answe is not A right ? Should be C as u said both N2 and O2 have same density

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  18. Hi,
    For Q34
    Why polymer is chemically inert which is wrong?
    Chemically inert means chemically unreacted?
    Thanks

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    1. MCQ34)
      ester bond of the polymer react with acid so it is not chemically inert.

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  19. For question 19, why Mg and Ca can be distinguished by using Br2 only ?

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    1. I think the word "only" is not very suitable. It should be deleted =]

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  20. HI,
    for q25
    The option" the oxidizing power increase from left to right in period 2" is correct.
    That means Neon has highest oxidizing power?
    Thanks

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    1. The sentence should have some problem.
      I will change it to "group I to group VII" =]

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  21. Hi, for q12, is that option 1 can be regarded as a neutralization, but its enthalpy change CANNOT be regarded as standard enthalpy change of neutralization since water is not produced? Thx very much

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    1. um...I think option (1) has some problem.
      NH3(aq) + HCl(aq) is neutralization because NH3(aq) should be written as NH4OH.

      But it is not standard enthalpy change of neutralization as you said.
      The energy of ionization of NH3 should be included in the standard enthalpy change of neutralization.

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  22. hi :)
    for q.27,i want to ask how to calculate the answer thx a lot

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  23. 1. convert the mole to concentration
    2. equilibrium expression
    3. sub the number and unknown to the expression which equal to the equilibrium constant 1.3

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  24. Q.7 why copper(II)chloride first but not sodium chloride?

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    1. MCQ07)
      because Cu is less reactive than Fe, displacement occur.
      Also, the concentration of ion in CuCl2 is higher than NaCl.
      Higher concentration of electrolyte speed up redox reaction

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    2. oh yeah didnt notice that Fe loses electrons more readily than Cu.. Thanks bro~~~~~~~~~

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  25. Q13
    why color around anode x turns yellow not red?

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    1. MCQ13
      X is cathode. H+ gain e become H2
      [H+] decrease, X become alkaline

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  26. how to calculate Q9 and Q16? QAQ

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    1. Fe2(SO4)3 + 3NaOH ---> 2Fe(OH)3 + ...
      mol of Fe2(SO4)3 = 0.015 dm3 x 2M =0.03mol
      mol of Fe(OH)3 = 0.03 x 2 = 0.06 mol
      mass of Fe(OH)3 = 0.06 mol x (55.8+17x3) = 6.41 g
      =C

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    2. MCQ16)
      2I- ---> I2 + 2e
      Fe3+ + e ---> Fe2+
      -----------------------------------
      2I- + 2Fe3+ ----> I2 + 2Fe2+

      [I-] drop 0.04M
      [Fe3+] drop 0.04M
      [Fe2(SO4)3] drop 0.04/2 = 0.02M

      so [Fe2(SO4)3] = 0.25 - 0.02 = 0.23 M

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    3. why [I-] drop 0.04M, [Fe3+] drop 0.04 but not 0.08? I give 2 mole of electrons while Fe3+ just gain one.

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  27. mcq1, I don't understand choice D.

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    1. the eqm constant of H2O --> H+ + OH-

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  28. why is q13 answer choice A? thanks!

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    1. I mean why Y is red, thanks

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    2. Y is anode, OH- ---> O2 + H2O
      so [OH-] decrease, [H+] > [OH-]
      and indicator change to red because of acidity

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    3. since it is very dilute KCl, so OH- discharge rathan than Cl-

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  29. dont understand q36?
    thank you

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  30. 作者已經移除這則留言。

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  31. Hi,in paper 1B Q(8), the equation of the decomposition of hydrogen peroxide to give oxygen should be 2H2O2-->2H2O+O2

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  32. Q17) why (3) is incorrect? isn't Mn2+ is very pale pink? it should be similar to pale pink..
    Q21) about (1), why is the mixture cloudy after Ca added to H2SO4? CaSO4 which is insoluble would form on the surface of Ca and prevent it from further reaction, so why is there a change in solution?

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  33. atomic radii 唔係out syl?

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    1. no 係 Patterns in the Chemical World 既野

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  34. Paper 1b question 8(b), the equation given is not balanced, so why not the answer be 0.036 dm3? Thx

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  35. About Q2, Option A
    OH- should be more preferentially discharge than Cl- in dilute HCL to give out O2吾係咩?
    Concentrated HCL will give out CL2 only, do I have sth wrong?

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  36. for q.7 why the ans is C instead of D ? isn't it no rusting occur for the hot oil case?

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  37. Q6
    In real life practice, silver cyanide is used as the electrolyte for silver plating instead of AgNO3. Moreover, silver sulphate is insoluble in water. Silver cyanide forms a complex ion [Ag(CN)2]- in the solution that preferentially discharges at the cathode

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